Abstract

The barrens-forming sea urchin Centrostephanus rodgersii (Diadematidae) has recentlyundergone poleward range-extension to Tasmania where grazing of diverse and economicallyimportant macroalgal beds has occurred. We compared growth, biometric, dietary and morphometriccharacteristics of C. rodgersii between macroalgal habitat and adjacent barrens to inform populationdynamics of the sea urchin within the newly extended range. The age structure of C. rodgersii inmacroalgal habitat and adjacent barrens is similar, suggesting that both habitats have been influencedby similar recruitment events. However, there are clear differences in body size, body mass,spine length and diet of sea urchins in the 2 habitat types. We identified 2 broad morphologies ofC. rodgersii. The first is of relatively large size with thick test, rapid growth and short spines, thatgrazes macroalgal beds (macroalgal eco-morph). The second ecomorph persists on the barrens habitatand demonstrates smaller body size, slower growth and thinner test, with gut contents dominatedby closely cropped filamentous and/or coralline algae; but occurs at higher density, possesses longerspines and appears to enhance the persistence stability of barrens habitat once it is formed (barrensecomorph). Invading dense, swell-prone macroalgal habitat of eastern Tasmania, the phenotypicplasticity displayed by this sea urchin appears to be an important mechanism facilitating colonisationof reef habitats within the extension region. Importantly, habitat-specific patterns in populationdynamics are broadly consistent with those observed from within the historical range, suggestingthat patterns in macroalgal–sea urchin dynamics and the ecological importance of this species will besimilar across the newly extended range.